Retroviral mediated gene transfer is the most efficient means to introduce new genetic material into the eukaryotic genome, and as such is a powerful tool for studying gene expression as well as a model for gene replacement therapy. Previously we showed that a modified N2 virus will transfer the human globin gene to erythroid cell lines and to primary hematopoietic progenitors. Infected mouse bone marrow cells were transferred to suitable recipient animals and transient beta globin expression was observed in some animals. We have analyzed 131 primative progenitor colonies (CFU-S), 58 of which contained an intact retrovirus, 56 of these (96%) expressed human beta globin mRNA at approximately 1% the level of mouse beta globin mRNA. Our previous conditions seemed to infect only progenitor cells rather than pluripotent stem cells. Therefore we have modified our infection procedure to include pretreatment of the donor animals with 5FU, and infection of the marrow in the presence of growth factors thought to promote stem cell cycling. Using the growth factor IL-3 alone approximately 1/3 of the CFU- S (Primitive progenitors) were infected and 3/31 mice were synthesizing human beta globin mRNA and protein 4 months post transplantation. Using a combination of IL-3 and IL-6, 77% of the CFU-S were infected and 8/21 mice were synthesizing mRNA and protein 4 months post-transplantation.